Figure 9. Electrical stimulation permits selective thalamic or cortical fiber activation. A, Thalamocortical sections of barrel cortex 3 weeks after injection of VPM with AAV expressing ChR2-mCherry. An electrolytic lesion (white cross) indicates the site of electrical stimulation. WM, White matter. B, Photoactivation of VPM terminals results in a pronounced negative field potential in L4. Black trace, Average of 50 trials without photoactivation; blue trace, average of 46 trials with photoactivation. Low-intensity electrical stimulation of the white matter (black arrowhead) follows 50 ms after a laser light pulse (blue arrowhead) and produces a comparatively weak fEPSP. Electrical stimulation artifacts are cropped for clarity. C, D, Average fEPSP (n = 40–48 trials per trace) in L4 evoked by electrical shock 50 ms after photoactivation of ChR2+ VPM presynaptic fibers (blue trace). Black trace, No prior photoactivation; black arrowhead, electrical shock onset. Insert, Configuration of the recording and stimulation electrodes in a coronal view of S1; ChR2+ fibers are indicated in blue. C, Electrical shock of white matter produces a fEPSP that is conditioned by prior (50 ms) photoactivation of VPM fibers (black vs blue traces). D, In the same animal, electrical shock of cortex L2/3 is not conditioned by prior photoactivation of VPM fibers. E, Percent change in fEPSP peak magnitude after photoactivation of VPM fibers. Left, Electrical shock in white matter; right, electrical shock in cortex L2/3 (magnitudes normalized to control amplitude); ctl, control condition (black traces in C, D); light, after photoactivation (blue traces in C, D). **p < 0.01, n.s. not significant; gray lines indicate individual animals; black lines indicate means.